Amiga Old File System: Difference between revisions

Revision as of 18:49, 31 August 2010

Revision as of 16:22, 19 September 2010

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== Characteristics ==

Amiga uses [[Modified Frequency Modulation|MFM]] encoding/decoding by default when handling [[floppy disk]]s. There are 80 cylinders on an Amiga floppy disk. Each cylinder has 2 MFM tracks, one on each side of the disk. Double density (DD) disks have 11 sectors per MFM track, Highhigh density (HD) disks have 22 sectors.

The geometry of an Amiga Floppyfloppy Diskdisk is as follows:

* DD disks: 512 bytes/sector, 11 sector/track, 2 track/cyl, 80 cyl/disk

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The standard Amiga filesystem has an internal 32 bit wide offset parameter (unsigned). It tells where to start the read/write operation. The biggest size for an Amiga disk is therefore 2³² = 4 [[gibibyte|GiB]]. This limit was changed with Amiga OS 3.5 and 3.9 and now is 2⁶⁴ = 4 [[exbibyte|EiB]].

An OFS Datablockdatablock stores Blockblock Sizesize BSIZE-24 bytes (i.e. normally 488 bytes at most frequently used BSIZE of 512 bytes), FFS stores BSIZE bytes (512 bytes per block).

FFS supports directory caching, links and international mode. The FFS is also faster than OFS.

The Rootblockrootblock is located at the physical middle of the media: block number 880 for DD disks, block 1760 for HDs.

The exact calculation for where it is stored is as follows:

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The Rootblockrootblock contains information about the disk: its name, its formatting date, etc. It also contains information on accessing the files/directories/links located at the uppermost (root) directory.

The characters '/' and ':' are forbidden in file and volume names, but *!@#$%|^+&_()=\-[]{}';",<>.? and letters with diacritical marks like âè are allowed.

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The date fields in the root block (and other blocks) are structured in the form of DAYS, MINS and TICKS. The DAYS field contains the number of days since January 1. 1978. MINS is the number of minutes that have passed since midnight and TICKS are expressed in 1/50s of a second. A day value of zero is considered illegal by most programs. Since the DAYS value is stored as a 32-bit number, the Amiga filesystem does not have an inherent [[Year 2000 problem]] or [[Year 2038 problem]].

To reach a file, directory or link in a removable media, Amiga has to compute its hash value with an algorithm often called a [[Hashhash function]]. After having computed the hash value, this is then used to access HashTable ('ht' field in Rootblock/Directory block). The HashTable[ HashValue ] contains the number of the first block of the object (File header block, Directory block or Link block).

It happens that different names can result in the same HashValue. If more than one name has the same HashValue, the other blocks (for files and directory only) are stored in a chained list. This linked list starts at the 'next_hash' field of the Filefile header or Directorydirectory block.

For example: '''file_1a''', '''file_24''' and '''file_5u''' have the same hash value.

There was a bug in the old AmigaDOS versions when applying the Hashhash function to international characters (ASCII codes > 128). Consequently, FFS was then created with the 'international mode' (INTL). Filename characters can be lowercase and uppercase, but are not case sensitive when accessed. That is to say, "MyFile" and "myfile" in the same directory refer to the same file.

Files are composed of a file header block, which contains information about the file (size, last access time, data block pointers, etc), and the data blocks, which contain the actual data. The file header block contains up to BSIZE/4-56 data block pointers (which amounts to 72 entries with the usual 512 byte blocks). If a file is larger than that, file extension blocks will be allocated to hold the data block pointers. File extension blocks are organised in a linked list, which starts in the Filefile header block ('extension' field).

== See also ==